(1) Field of the Invention
The present invention relates to acoustic signal conditioning and more particularly to a means for detecting an induced voltage noise function due to local turbulent flow in combination with a piezoelectric hydrophone for detecting pressure variations due to both the local turbulent flow and imbedded acoustic signals and correlating, processing and conditioning the detected noise function and pressure variations in such a way as to substantially enhance the imbedded acoustic signals.
(2) Description of the Prior Art
It is well known that piezoelectric hydrophones are used to measure acoustic signals by producing a voltage signal corresponding to the elastic deformation of the piezoelectric components caused by pressure variations at the hydrophone face. The measurement of acoustic signals utilizing piezoelectric hydrophones suffers from the fact that the strong pressure variations due to local turbulence at the hydrophone face dominate, or mask, the relatively weak acoustic signals of interest. To reduce the effects of these unwanted pressure variations, or noise, a number of noise conditioning techniques are used to enhance the signal to noise ratio and obtain meaningful measurements of acoustic signals. However, as the fluid velocity across a hydrophone increases, the noise component increases, thus limiting the velocity envelope within which useful acoustic signal measurements can be made.
It is further well known that electromagnetic turbulent velocimeters measure the turbulent velocity structure, as illustrated in U.S. Pat. No. 4,848,146. The electric potential difference generated by a conductive fluid passing through a magnetic field is sensed by orthogonally arranged electrodes in the velocimeter. The velocimeters respond exclusively to the velocity structure and have been shown to be insensitive to far or near field acoustic excitation. As such, the velocimeters cannot be used to measure acoustic signals.